The object of this invention is to reduce or prevent the fading of dyed nylon fabrics caused by the presence of ozone. By fading is meant the decrease in concentration of a dye in the fiber such that the fiber becomes less intensely colored or changes color, for example, dark blue to light blue, or green to yellow because of the loss of a blue dye.
Ozone is normally present in air at sea level at concentrations of about 1 to 5 parts per hundred million. Despite this extremely low concentration, severe fading, caused by ozone, has been observed.
Ozone is an allotropic form of oxygen. The molecule of ordinary oxygen consists of two atoms of oxygen whereas the molecule of ozone is formed from three atoms of oxygen. Ozone is created in the upper atmosphere by the action of high energy radiation from the sun splitting oxygen molecules into separate oxygen atoms. The individual atoms then combine with diatomic oxygen molecules (O.sub.2) to form triatomic ozone (O.sub.3). This ozone then diffuses down through the atmosphere.
Ozone is a vigorous electrophilic reagent, that is, it acts by sharing electrons which previously belonged exclusively to another molecule (see C. K. Ingold's "Structure and Mechanism in Organic Chemistry", Cornel University Press, 1953, page 201). An example of this sharing is the attack of ozone on a carbon-carbon double bond to form an ozonide.
When ozone attacks a dye, it can attack at a double bond in the dye or at an amine nitrogen, or other sites where there are electron pairs available. The dyes which are attacked are usually anthraquinone type dyes. Lebensaft in his doctoral dissertation (University of North Carolina at Greensboro, 1970) stated that ozone attacks the anthraquinone nucleus to form derivatives of phthalic acid.
Disperse type dyes and cationic dyes are those most frequently attacked, but under severe conditions, it is believed that almost all dyes are affected by ozone.
One of the most sensitive dyes to ozone fading is Disperse Blue 3. The major constituent of Disperse Blue 3 has the structure: ##SPC1##
It is believed that the dye diffuses through the fiber to contact the ozone at the surface of the fiber, rather than ozone diffusing through the fiber to a relatively immobile dye. Part of the evidence for this is that any physical treatment to the fiber that increases the mobility of the dye, increases its ozone fading. For this reason, it is felt that any protective agent must also be able to diffuse through the fiber, if it does not form an impenetrable film on the surface of the fiber.
There is some evidence that water molecules must be present in the nylon fiber in order for ozone fading to take place, but whether its action is that of a swelling agent or it is involved in a primary or secondary oxidation step is unclear. Lebensaft, cited above, believes it functions principally as a swelling agent and a carrier.
Ozone fading can be decreased by reduction of the specific surface of the yarn. This reduction is undesirable in end-uses that need a bulky yarn. Ozone fading also can be diminished by changes in polymer morphology and orientation, but these techniques are inherently expensive.
Prior methods of improving ozone fading resistance either affect the rate of dyeing, the leveling properties of the dye-fiber system, or have poor dyed light-fastness, at least in some shades.